Position determination in lithography systems is generally known, normally using detection of light reflected in several diffraction orders, as is described for instance in U.S. Pat. No. 4,967,088. A disadvantage of determining a position using light reflected in several diffraction orders is that the light detectors for the different diffraction orders have to be accurately positioned in the system, thus increasing the cost of the system. Moreover, such systems are sensitive to slight errors in focus of the light beam or tilt of a substrate relative to the light beam.
U.S. Pat. No. 5,827,629 discloses a similar system, wherein a wafer with an exposure surface and an exposure mask are disposed. The exposure surface is directed to the exposure mask with a gap being interposed therebetween. The wafer has a position aligning wafer mark formed on the exposure surface. The wafer mark has a linear or point scattering source for scattering incident light, and the exposure mask has a position aligning mask mark having a linear or point scattering source for scattering incident light. A relative position of the wafer and exposure mask is detected by applying illumination light to the wafer mark and mask mark and by observing scattered light from the scattered sources of the wafer mark and mask mark. In order to at least partially overcome the problems of the above-mentioned prior-art, it has been suggested by the inventors to provide a substrate comprising a checkerboard pattern of reflective squares, having a maximum reflection coefficient, and non-reflective squares, having a minimum reflection coefficient, wherein said squares have a width corresponding to a diameter of a cross-section of a light beam projected on said pattern. By measuring a reflected zero-th order intensity of the beam, a change in position of the beam relative to the substrate can be determined without measuring multiple diffraction orders. Ideally, when the beam spot of the light beam is moved over the pattern, the intensity of the reflected signal is a sinusoidal function with high contrast of the position of the beam spot on the pattern. However, in practice the intensity distribution of the beam spot generally does not correspond to a homogeneous and sharply cut-off disc-like profile but instead follows a Gaussian profile, the resulting reflected intensity signal does not closely resemble a sinusoid function as a function of the position of the beam on the substrate. As a result, the determination of the position of the beam spot on the substrate based on the intensity of the reflected beam is less accurate.
U.S. Pat. No. 7,418,125 B2 discloses an apparatus for detecting a position of a region, corresponding to a mark, in image data, as a mark position, the mark including periodically arranged patterns. A first unit obtains a real-space energy distribution which corresponds to an energy spectrum distribution of each partial area of the image data. A probability distribution obtaining unit obtains a probability distribution based on the real-space energy distribution, the probability distribution indicating repetitive positions of the periodically arranged patterns and the intensity of periodicity at this position. A second unit obtains a degree of correlation between each probability distribution obtained by the probability distribution obtaining unit and a pre-registered probability distribution of the mark, and a third unit obtains the mark position based upon the degrees of correlation obtained by the second unit.
All of the above-mentioned methods of determining a position of a substrate in a lithography system are cumbersome, complex, and suffer from draw-backs, such as inaccuracy.